During processes such as chemical vapour deposition processing, process gases are supplied to a process chamber to form a deposition layer on the surface of a substrate. As the residence time in the chamber of the process gas is relatively short, only a small proportion of the gas supplied to the chamber is consumed during the deposition process. The unconsumed process gas is subsequently pumped from the process chamber with one or more by-products from the process using one or more vacuum pumps.
The gas stream pumped from the process chamber can contain species that may cause premature failure of the pump. For example, some deposition process generate particulates which are exhaust from the process chamber with the unconsumed process gases. These deposits can accumulate within the pump and effectively fill the vacant running clearance between the rotor and stator elements of the pump, leading to a loss of pumping performance and ultimately pump failure.
As another example, many semiconductor processes use or generate solid, condensable or subliming compounds. For example, low-pressure chemical vapour deposition silicon nitride (LPCVD nitride) processes tend to use chlorosilanes (such as dichlorosilane or trichlorosilane) and ammonia to produce a uniform layer of silicon nitride to insulate a substrate. By-products of this process include complex ammonium-chloro-silicate salts, for example, ammonium hexachlorosilicate, which sublimes at 120° C. at atmospheric pressure. If the unconsumed process gas or by-product is condensable, sublimation on lower temperature surfaces can also result in the accumulation of powder or dust within the vacuum pump.
Furthermore, the gases exhaust from other deposition processes can contain unreacted species, such as tungsten hexafluorate or copper precursors used in the CVD of a tungsten or copper film on a substrate. The temperature and pressure within the pump can cause these unreacted species to react, forming solid material that coats the rotor and stator elements of the pump.
In view of this, there are a number of existing designs for trap devices for capturing species entrained within a gas steam. To capture particulates entrained within the gas stream, a trap device incorporating standard filter elements is located upstream from the vacuum pump so that the particulates become trapped within the filter elements. To capture condensable species contained within the gas stream, a cold trap device is typically provided at the outlet of a pump heated to a temperature above which the condensable species will pass through the pump without condensing within the pump. Such traps typically comprise a water-cooled coil located within a flow passage of the trap. As the gas stream flows through the flow passage, it contacts the coil, which cools the gas stream and causes low boiling point species within the gas stream to condense inside the trap.
A problem associated with each of these trap devices is that the device can become rapidly blocked by the collected solid material, causing a significant reduction in pump performance. As a result, the device must be periodically serviced to remove the solid material from within the trap. As the gas stream passes through the trap device as it flows towards, or away from, the vacuum pump, the gas flow must be temporarily stopped in order to allow the trap device to be emptied and cleaned, incurring down time and loss of production. Furthermore, the person cleaning the trap becomes exposed to the collected material, which, depending on the chemistry of the collected material may be particularly hazardous.
In addition, by heating the pump, the temperature of the gas stream may be heated to a temperature above which unreacted species within the gas stream are converted into solid material. For example, tungsten hexafluoride passing through a hot pump can form deposits of tungsten within the pump, which can lead to damage of the pumping mechanism.
It is an aim of at least the preferred embodiments of the invention to provide a trap device connectable to the inlet of a vacuum pump and which can enable rapid and safe servicing thereof.